1. Field of the Invention
This invention pertains generally to the field of slow-wave structures for a traveling-wave tube (TWT), and more particularly, to re-entrant ladder-type coupled-cavity circuits with periodic permanent magnet (PPM) focusing having direct liquid cooling of the beam tunnel.
2. Description of the Background Art
Coupled-cavity TWT structures are advantageously and widely utilized in the design of high-power wide-band amplifiers. In the "ladder-type" coupled-cavity circuits, the periodic interaction elements resemble the rungs of a ladder extending across a hollow tube. The spaces between the rungs constitute the cavities, and coupling apertures between adjacent cavities are defined by the spaces around the rungs. The bandwidth of the structure increases with increasing intercavity coupling. By providing PPM focusing of the beam, it is possible to design a compact lightweight structure having the above advantages of high power and good bandwidth characteristics. Such TWT's combine the PPM periodicity with that of the RF circuit, the magnetic circuit forms a part of the cavity structures.
One typical ladder structure is disclosed in U.S. Pat. No. 4,409,519 issued Oct. 11, 1983 to Arthur Karp. This patent discloses a structure with wide rungs and coupling apertures staggered on alternating opposite sides of the rungs so that each cavity is coupled only to its immediately neighboring cavities. This staggered coupling increases the usable bandwidth of the structure. The wide rungs also allow heat conduction in two dimensions away from the beam tunnel, thereby improving the thermal properties of the tube.
A double staggered ladder circuit is disclosed in U.S. Pat. No. 4,586,009 issued Apr. 29, 1986 to Bertram G. James, who is also the present inventor. This structure includes two coupling apertures between each pair of adjacent cavities. The relative locations of these apertures are rotated by 90 degrees about the beam axis in successive intercavity interfaces. This double coupling further increases the bandwidth of the system.
Another type of double coupling is disclosed in U.S. Pat. No. 4,866,343 issued Dec. 2, 1980 to Arthur Karp. This structure is the "comb-quad" circuit, which comprises two mutually orthogonal ladders with their rungs interleaved. There are construction difficulties in aligning the components of this structure. Further, the heat conduction away from the beam tunnel occurs essentially in one-dimension along the rungs or "teeth" of the comb, and this limits the average power at which the tube can operate.
A further improvement on the double-staggering design is disclosed in U.S. Pat. No. 4,866,343 issued Sep. 12, 1989 to Bertram G. James, the present inventor. This improvement is termed the "Re-Entrant Double-Staggered Ladder Circuit", in which each plate or "wall" between adjacent cavities has a wide transverse ridge on either side enclosing the axial beam aperture. The ridges are orthogonal to the coupling slots in these walls, and the slots and ridges in neighboring plates are rotated by 90 degrees about the beam axis relative to each other. These re-entrant ridges increase the efficiency and bandwidth of the traveling wave tube.
All of the foregoing cited United States patents are assigned to the assignee of the present invention.
In all of the traveling-wave tubes discussed above, the average power capability is limited by the heat generated from the interception of the electron beam by portions of the RF structure. This heat must be conducted away from the beam by the structure, and therefore the structure must have good thermal conducting properties to maximize the operating power of the tube. Copper is often used in these structures because of its high thermal conductivity.
In coupled-cavity PPM TWT's, heat is generated by the electron beam interception in the iron pole pieces, which have lower thermal conductivity than copper. In order to improve the thermal conduction path away from the electron-beam tunnel, a ferrule bar may be utilized, as described in the article by Alan Griggs entitled "A New Coupled-Cavity Circuit for High Mean Power Traveling-Wave Tubes", IEEE Transactions on Electron Devices, Vol. 38, No. 8, Aug. 1991, pp. 1952-1957. This ferrule bar is essentially a high-conductivity copper bar extending from the iron ferrule around the beam to the outer copper cavity wall, which is in contact with coolant channels. The author states that this technique is useful at operating frequencies that exceed 4 GHz, the maximum frequency at which direct liquid cooling of the beam tunnel is feasible. For frequencies greater than 4 GHZ, the article reports that the ferrule bar technique improves the mean power capability of the tube by a factor ranging from 1.5 to 3, depending on the frequency.
When the frequency substantially exceeds approximately 30 GHz, the intercavity walls become too thin to serve as magnetic pole pieces. The magnetic circuit is then made external to the RF structure, and the cavity walls are made of copper. In this high frequency region, the increase in available mean power from the ferrule bar technique would be less significant, but it is still useful.
Other designs have been utilized to increase the thermal capacity of PPM coupled-cavity circuits. These designs include laminated plates of copper and iron serving as the pole pieces, and the use of water channels and heat pipes through the pole pieces. These techniques are useful to some degree, but they are limited either because of mechanical restrictions in that thick pole pieces are required, or because in tubes operating at high frequencies the structure must be made so small that the design is not practical.